1. Technical Field
The present invention relates, generally, to transmission of data over telephone lines using a modem, and more particularly, to sending time critical data, such as interactive data, over a modem link with low latency.
2. Background Art and Technical Problems
Modern modem modulation schemes used for transmission of data over telephone lines typically employ an error correction process to improve transmission characteristics and throughput of data. Error correction is generally required to reduce block error rates to acceptable levels for reliable bulk data transmission across a typical telephone line link.
Modem based compression is also employed almost universally to increase effective data throughput speeds. Typical telephone lines such as the public switched telephone network, or PSTN, have a limited bandwidth that limits the speed at which data may be sent over the telephone lines. Compression reduces the number of bits that must actually be transmitted over a link of limited bandwidth, and the compressed data stream is decompressed at the other end of the link to recreate the original data. Using compression, more data can be effectively communicated across a link in less time, thereby increasing the effective speed at which communication takes place. For example, using compression, the time that it takes to download a file over a telephone line may be significantly reduced as compared to the same file downloaded with no compression.
However, error correction processes and compression are typically performed in a V.42 type scheme that adds significantly to the amount of delay experienced over the link in communicating data. The amount of data buffered in a V.42 compliant scheme introduces delay into the link which can be significant when time critical data is being transmitted over the link. In addition, a finite amount of time may be required to perform the error correction process on both sides of the link. Similarly, the data compression and decompression that must be performed on the data will consume additional time, and may contribute to the link delay.
Problems arise when multimedia or interactive data are being sent across a modem link. For example, in voice over IP (or Internet Protocol) applications, the latency added by error correction schemes and data compression may present a significant quality problem when time critical data is being transmitted over the link. The link delay may cause sluggish performance and slow response time in an interactive environment. While error compression schemes and data compression may have significant advantages in other environments, the link delay caused by such techniques has caused serious problems in applications where low latency is a requirement. Improved techniques are needed for the transmission of time critical data over modern links.
In the past, voice has been digitized in a modem, and the audio data was recovered directly at the other end of the modem link. This solution does not work satisfactorily, however, in an IP centric application, where the voice data is further transmitted over a network beyond the modem link itself.
Link delays may be a significant problem in applications where data is transmitted over a modem link, and then further transmitted over a network using an Internet protocol, e.g., an IP network. For example, a dial up connection to an Internet service provider, or ISP, who provides user connections to the Internet, presents such an environment. Data must be transmitted over a telephone line link between the user""s computer and the location of the ISP server. Modems at both ends of the telephone line link are employed to transmit the data. The user is connected to the Internet via the ISP server, and the data is further transmitted over the IP network, perhaps to an Internet site at a great distance from the user""s location. There are no known solutions for applications such as voice over IP where it is required to send data with low latency across a telephone line link connecting two modems, as well as across an IP network to a remote destination that may be as far away as another continent.
Thus, efforts to develop schemes for the transmission of data over modem links have not been altogether satisfactory, particularly where interactive data or other time critical data is being transmitted, and significant room for improvement still exists.
In accordance with an exemplary embodiment of the present invention, a method and apparatus for transmission of time critical data over a modem link is provided. Time critical data is intercepted by the modem at its host input-output port. The intercepted time critical data is injected directly at the top of the queue of data intended for transmission over the modem link. The injection of time critical data takes place at the input to the modulator in the form of specially identified frames, preferably of short duration. Time critical data is thus sent on a priority basis bypassing error correction circuits or routines, and bypassing data compression.
At the receiving modem, time critical data is recognized in the demodulator as specially identified frames or packets. Such time critical data is recovered and immediately sent to the host without passing through the error correction process and without decompression.
The present invention removes a significant amount of delay over modem links and substantially contributes to the quality of applications where time critical data must be timely processed, such as in voice over IP applications using modem links.